General behavior of cohesive soils Cohesive soils (e.g., clays) typically exhibit which combination of mechanical characteristics under normal engineering conditions?

Introduction / Context:
Classifying soils by mechanical behavior guides selection of tests and design methods. Cohesive soils (clays and plastic silts) are distinct from sands in their plasticity and compressibility due to plate-like particles, interparticle forces, and water chemistry effects.

Given Data / Assumptions:

• Normal ranges of stresses encountered in geotechnical projects.
• Typical natural water contents near or above plastic limit for clays.
• Time-dependent behavior (consolidation) acknowledged.

Concept / Approach:

Clays are plastic over a range of water contents (between plastic and liquid limits), allowing them to undergo significant remoulding without cracking. They are also compressible, exhibiting primary consolidation under sustained load as pore water flows out, followed by secondary compression (creep). Elastic behavior is only a small portion of the overall response.

Step-by-Step Solution:

Verification / Alternative check:

Laboratory tests (Atterberg limits, consolidation tests) consistently show plastic deformation ranges and significant volume change under load for cohesive soils.

Why Other Options Are Wrong:

“Elastic and compressible” overemphasizes elasticity; “plastic but incompressible” is contradictory to observed consolidation; “none” and “rigid/impermeable” are inconsistent with field behavior.

Common Pitfalls:

Assuming clays are impermeable or incompressible; ignoring time effects and drainage conditions in analysis.

Final Answer:

Plastic and also compressible